Matching of the local area-specific gas flows in PEM fuel cells

Abstract

A passage structure for PEM fuel cells is configured locally differently in order to adapt the flow of gas on the membrane electrode assembly (MEA). Starting from the cathode entry port, the active gas volumetric flow per unit area on the MEA is locally varied by changing the number and/or cross section of the gas-carrying passages, so that the flow decreases toward the cathode exit port. Suitable configuration of the passage structure according to the invention allows the steam partial pressure to be optimally matched to the local conditions. The mass conversion is improved in these regions as a result without the electrolyte's drying out. At locations in the flowfield with a low gas humidity, the measures according to the invention (increased number of passages or passage width) significantly reduce the flow velocity. As a result, the transfer of water from the MEA to the gas flow is reduced and drying of the MEA is diminished.

1. A fuel cell having a membrane electrode assembly with a flowfield situated within an active cell area, through which flowfield reaction gases flow in communication with the active cell area; wherein:
 said reaction gases are passed through passages within said flowfield, from an entry port of said flowfield to an exit port of said flowfield; and 
 passage volume of said passages through which the reaction gases flow in communication with said active cell area is decreased in a direction of flow, by reducing successively within the active cell area the number of passages through which the reaction gases flow in parallel, in sections. 
 
     
     
       2. The fuel cell according to  claim 1 , wherein the passage volume is further decreased by reducing cross-sectional area of each of said passages. 
     
     
       3. The fuel cell according to  claim 2 , wherein the reduction in the passage cross-sectional area takes place in steps, from the entry port to the exit port. 
     
     
       4. The fuel cell according to  claim 2 , wherein the reduction in passage cross-sectional area takes place continuously, from the entry port to the exit port. 
     
     
       5. The fuel cell according to  claim 1 , wherein the passage volume through which the reaction gas flows per passage unit length is increased on a cathode side at locations of an active cell area at which relative humidity of the flowing gases is lower than in remaining regions of the active cell area. 
     
     
       6. The fuel cell according to  claim 1 , wherein the passage volume through which reaction gas flows per unit passage length is increased on an anode side at locations of an active cell area at which relative humidity of the flowing gases is lower than in remaining regions of the active cell area. 
     
     
       7. A fuel cell having membrane electrode assembly and comprising:
 an entry port; 
 an exit port; 
 a plurality of gas flow passages connecting said entry port to said exit port; wherein, 
 a total cross sectional area of said gas flow passages decreases in a direction of gas flow from said entry port to said exit port; 
 each of said gas flow passages has a uniform cross sectional area along its entire length; and 
 a number of gas flow passages comprising said plurality decreases in said direction of gas flow. 
 
     
     
       8. A fuel cell having a membrane electrode assembly comprising:
 a flowfield in which reaction gases flow within an active cell area of the fuel cell; 
 passages within said flowfield for accommodating said flow of reaction gases, from an entry port to an exit port; 
 wherein an aggregate passage volume of said passages within said flowfield is decreased in a direction of flow, by successive reduction of the number of said passages within the active cell area through which the reaction gases flow.